The present invention relates to a color image forming apparatus that is represented by apparatuses, such as an electrophotographic-type color electrophotocopying apparatus and a color printer that form color images by superimposing monochromatic images.
Most existing color image forming apparatuses employ a print method such as that a predetermined number of monochromatic-image forming units is serially aligned, and monochromatic images formed by the individual monochromatic-image forming units are superimposed on a sheet material. These monochromatic-image forming units includes a photosensitive unit and a developing unit. The number of the monochromatic-image forming units is defined by the number of subtractive primaries (ordinarily, the number thereof four). With these monochromatic image forming units, images of subtractive primaries, yellow (Y), magenta (M), cyan (C), and black (B) (for reinforcing the darkness) are sequentially superimposed on a sheet material. The sheet material is transported by a sheet material transfer belt provided along the individual image forming units. The transfer belt is designed for transporting sheet materials including transparent resin sheets for use with overhead projectors and sheet papers. For superimposing the four images, there is also known a method that allows the four images to be transcribed on an intermediate transferring unit and allows the four images superimposed thereon to be transcribed at the same time.
In these color image forming apparatuses, four color images (four images) are required to be accurately superimposed. Therefore, various types of control are employed to obtain accurately superimposed images.
For example, the control includes the photosensitive unit peripheral speed control and belt peripheral speed control. The photosensitive unit peripheral speed control controls a drum-driving motor to rotate at a constant speed so that a photosensitive-unit peripheral speed, by which an arbitrary point on a peripheral surface of the photosensitive unit provided in each of four photosensitive units is moved, is the same as the belt peripheral speed, by which an arbitrary point on a sheet material transfer belt rotated by a belt driving motor is moved. The belt peripheral speed control detects the rotation speed of a transferring-belt driving motor and thereby controls it to be constant so that the photosensitive-unit peripheral speed is the same as that of the belt. Also, correction is included in the control to be performed for spacings at which portions where the individual photosensitive units of the four image forming units in contact with the sheet material transfer belt. The correction is performed by changing image forming timing for portions where the images are superimposed.
However, in actual operation, it is difficult to obtain a superimposed image that is completely free of deviation for various reasons. The reasons include positional deviations occurring when exposure light is incident on the individual photosensitive units, deviations in pitch of the photosensitive units (image forming units), slippage occurring between a driving roller for driving the sheet material transfer belt and the sheet material transfer belt, the variation in the peripheral speed of the sheet material transfer belt because of changes in the diameter of the driving roller due to thermal expansion.
For these reasons, a run-in control sequence for converging color deviations and an image-density control sequence are also included in the control. The run-in control sequence converges color deviations occurring when images are superimposed. This control is carried out at power-on time and using warm-up time after a cover or the like is opened or shut after sheet materials are jammed or stacked in the apparatus. The image-density control sequence serves to maintain the image density (toner adhesion amount) even when characteristics vary according to the variation in temperature and aged deteriorations.
With the described various corrective control operations being provided, however, color deviation (positional deviation in the superimposed image) occurs. This color deviation occurs when the difference occurs between the peripheral speed of the sheet material transfer belt and the speed of transportation of sheet materials that are transported by an aligning roller toward the sheet material transfer belt.
For example, when the transportation speed of the aligning roller is lower than the peripheral speed of the sheet material transfer belt, the aligning roller causes a load to exert on the sheet material transported by the sheet material transfer belt. The load is exerted in the direction opposing the direction in which the sheet material is transported; therefore, the sheet material is pulled in the aforementioned direction, color deviation occurs on the whole of the sheet material. Also, jitter is caused because of influence of oscillation in paper-feed driving system, which is transferred from the aligning roller.
In contrast, when the transportation speed of the aligning roller is higher than the peripheral speed of the sheet material transfer belt, great deflection occurs on the sheet material. The deflection occurs in a space defined by upper and lower guides provided so as to sandwich the sheet material from the upper and lower sides. That is, the deflection occurs in front and back portions in the direction in which the sheet material is transported between the aligning roller and a roller provided for electrically charging the sheet material is electrostatically attracted onto the sheet material transfer belt. When the deflection of the sheet material increases to a level that cannot be incorporated in the aforementioned space, the deflection extends and thereby causes the sheet material to shake (or, to wave) in the direction in which the sheet material positioned on the sheet material transfer belt is pushed. This causes the position of the sheet material placed on the sheet material transfer belt to deviate, thereby causing color deviation as in the earlier case.
Thus, an appropriate speed range is narrow for the sheet material transportation speed required for the aligning roller; therefore, the speed must be neither excessively high nor excessively low. An image forming apparatus dedicatedly handling the monochromatic color (black) does not perform color-superimposing, and an image forming apparatus performing digital processing represents the density gradation in the density of pixels by performing binary-coding processing. Therefore, in either of these image forming apparatuses, jitter does not easily occur. However, in the color image forming apparatus, color-superimposing must be performed, and in addition, the density gradation is obtained by multivalue processing, in which pixels having sizes differing from each other are formed at uniformed pitches, thereby causing noticeable jitter.
The color image forming apparatuses as described above also includes multiple image (position) sensors for converging color deviations occurring when images are superimposed. Also, it has an image-density control sensor that serves to maintain the image density (toner adhesion amount) even when characteristics vary according to the variation in temperature and aged deteriorations. In addition, the color image forming apparatus includes an identifying sensor for overhead projector (OHP) sheets.
With this application, the inventor proposed a technique that calculates the sheet material transportation speed, as described above, thereby allowing control of the number of revolutions of the driving roller, which drives the sheet material transfer belt, and the peripheral speed of the photosensitive-unit drum.
However, the proposed techniques, which controls the number of revolutions of the driving roller, which drives the sheet material transfer belt, and the peripheral speed of the photosensitive-unit drum, still arises a problem. The problem is that, for OHP sheets as sheet materials, detection outputs produced by individual sensors contain many errors, and the OHP sheets cannot be identified.
An object of the present invention is to provide a color image forming apparatus that adjusts relative differences in sheet material transportation speeds of a sheet material supplying unit and image forming units to be within an appropriate relative speed range and that, particularly, prevents color deviations from being occurred in a color mode.
Another object of the invention is to provide a color image forming apparatus that allows transparent sheets for use with OHPs to be accurately identified.
Still another object of the invention is to provide a color image forming apparatus that accurately detects test images, thereby allowing forming of color images even for OHP sheets, without causing defects such as variation in the output image density and deviation in timing for image forming, as in the case of ordinary sheet papers.
The present invention provides an image forming apparatus comprising: image forming means for forming images by using developer; supplying means for feeding image formation media toward the image forming means; transporting means for transporting the image formation media that have been fed; transferring means for transferring developer images formed on the image forming means onto the image formation media; identifying means for performing identification of the type of the image formation media; transportation speed adjusting means for performing adjustment of the speed of transportation by the supplying means for the image formation media, the adjustment being performed according to outputs from the identifying means; and identification reference adjusting means for adjusting a reference value E used for identification of the image formation media, the identification being performed by the identifying means, wherein a sequence for adjustment of the reference value E is executed by the identification reference adjusting means, and identification is thereby implemented for the type of adjustment sequences that are executed by the transportation speed adjusting means for the speed of transportation of the image formation media and the type of the image formation media.
In addition, the invention provides an image forming method for feeding image formation media toward an image forming apparatus and forming images on the image formation media, comprising: a step of feeding a first adjustment sheet to an identifying means provided for identifying an image section and a non-image section formed on the image formation media, the first adjustment sheet having a predetermined image and being used for adjustment of a reference value with which the identifying means performs identification; a reading step of allowing the identifying means to read the image section of the first adjustment sheet and the non-image section of thereof; a reference value adjusting step of adjusting the reference value of the identifying means according to a read value regarding the image section and a read value regarding the non-image section; a step of feeding a second adjustment sheet to the identifying means of which the reference value has been adjusted in the reference value adjusting step, the second adjustment having a predetermined image and being used for adjustment of a speed at which the image formation media are fed to image forming means; a speed adjustment step of adjusting a feeding speed for the image formation media according to results of the identification performed by the identifying means for the image section of the second adjustment sheet and the non-image section thereof; and a step of feeding the image formation media to the image forming means at the speed adjusted in the speed adjustment step and forming images on the image formation media that have been fed.